Hi folks
Modified BIOS has been tested successfully on four T60, one X61t and one R32. It's been five days since first T60 was flashed, nine since R32, and everything works as expected. I consider the problem
solved. These machines allow any WWAN/WLAN card to be inserted and won't complain with 1802 or 1804 error.
I've invested a lot of time in it and have to take care of my other tasks too now, so maybe next week, I'll create a tool to modify the BIOS image and share it with others. Ideally, you'll have just to take .FL1 file from the BIOS update, run it through the program and put the modified file back to the CD, perhaps using MagicISO (as I've done).
I have working BIOS images for T60, X61t and also R32, you can ask for these in the meantime (PM). X60 needs a change in module size, so that will come later after I do some more testing.
I would like to thank everyone who has helped, specifically bonnem, for allowing me to do a proof-of-concept changes to the BIOS on his T30, Jumme for being the first tester with machine in daily use, maimbourg for bearing with my inability to calculate checksum and short101 for a nice talk and information about generic MC8775.
Mission accomplished.